Why do we apply paint on iron articles?

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Slowly eating away of metal due to the attack of air, water, moisture, and chemical present in the environment is known as corrosion.
Corrosion of iron particles is called rusting. When iron reacts with water air and oxygen presents in the environment it forms a reddish-brown layer on it which is called rust and this process is called rusting.

Complete answer:
 Chemically rust is an Iron oxide layer formed on iron articles. When rusting develops on iron articles they become poisonous and they look dull.
The formation of rust requires iron, water, and oxygen. Although it's a complex process, the chemical equation is simply $4Fe$ + $3O_2$ + $6H_2O$ → $4Fe(OH)_3$
In order to prevent the rusting of iron we apply paint on iron articles so that the layer of iron can not come in contact with the environment and do not react with oxygen to form iron oxide and rust is not developed.
Another way to prevent it is to oil the iron articles so that water or oxygen does not come in contact with air and water. Linseed oil helps to remove the rust from the iron articles. Any acrylic paint helps to prevent the rusting of iron.

Note: Corrosion of silver is due to the silver sulphide layer formed on its surface. The silver becomes blackish in colour due to its corrosion.
-Corrosion of copper is due to the oxygen and carbonates due to which it forms a copper oxide which is red and black in colour and copper carbonate which is green in colour.
-Corrosion of steel and iron is due to oxygen due to which it forms an iron oxide on its surface which is reddish-brown in colour.